1. Field of the Invention
This invention pertains to a small and portable power source, and specifically pertains to a thermophotovoltaic power source that generates heat bringing an emissive spectrum converter to incandescence and thereby producing light in a narrow wavelength spectrum to which nearby photovoltaic cells are particularly sensitive, generating electricity to power portable and hand-held devices which require significant amounts of energy but are presently limited in use because of the low energy densities provided by conventional power sources.
There are a variety of civilian and military situations which would benefit from a portable power source that can supply larger amounts of energy over extended periods of time than conventional batteries. Electronic surveillance systems, robotics, communications and computing devices all have very limited periods of usefulness when they are mobile or must necessarily be disconnected from power grid connections. A power source capable of delivering higher energy densities would not only provide extended life for the systems above, but would enable other devices requiring higher input energies to operate remotely.
2. Prior Art
Our increasing abilities to make use of electronics technology has created a need for being mobile with our technological devices. As technology advances, these devices become capable of accomplishing more tasks. However, portable devices have limited use because conventional batteries quickly lose their charge. Furthermore, our mobil society has only increased our desire to be able to handle any task requiring technology, no matter where we are. Consequently, devices are developed to provide remote location capabilities that can rival those of a home or office. Nevertheless, with increased capability comes the inevitable increase in power requirements. Unfortunately, portable power for devices such as cellular phones, notebook computers, power tools, toys, medical equipment, radios, pager, backup power supplies and robotic applications has always meant batteries. Batteries come in many shapes and sizes, but share several characteristics which limit their use.
For example, alkaline, nickel-cadmium and nickel-metal hydride batteries commonly used in the above mentioned devices all depend upon chemical energy being converted into electrical potential energy. The energy density of these batteries is low so as to prevent or limit the explosive discharge of energy if the battery cells should be ruptured, compressed or heated. Although it is possible to create better batteries, tests show that batteries with an energy density similar to that of explosives tend to act like high explosives when mistreated.
Low energy density translates into a battery providing a small amount of power over a relatively short time period for the user. But there are other drawbacks to conventional batteries as well. Even if the battery can be recharged, the time it takes to recharge is probably time lost being unproductive, especially when the batteries must be returned to a home site for charging. Adding to the problem is the cost of equipment necessary for recharging. Furthermore, the shelf-life of batteries is also only a matter of years, and batteries can lose their charge while waiting to be sold. Another realistic concern these days is the environmental impact of disposed batteries and the corrosive elements that leak during decomposition.
Attempts have been made to provide more efficient portable power sources that do not suffer from all the drawbacks of batteries. Most of the attempts have focused on improving features of batteries themselves, such as substituting the chemicals storing energy. Other alternative power sources have been developed for commercial use, but have not been miniaturized to replace batteries because of technological barriers that have, until now, prevented this action. Such a power source is thermophotovoltaic (TPV) technology.
TPV power is generated by elevating a radiant emitter to incandescent temperatures. The radiant energy is captured on semiconductor photovoltaic converter arrays (solar cells) and converted directly into electrical power to be stored or used immediately. While this principle of operation sounds simple enough in macro applications, the problems of miniaturization have prevented anyone, as far as the inventors of the present invention are aware, of developing and successfully implementing a miniaturized TPV power source.
For example, up to the present time, TPV devices have been limited exclusively to macro-applications. That is, the size of the TPV power sources has been relatively large. TPV power sources have been proposed for such tasks as generating energy to be stored for household purposes. A TPV device could be used to provide lighting and heat water, and the electrical energy produced as a by-product of the process could be stored in batteries and accessed for use by household appliances such as a heating fan.
The basic elements of a TPV power source have largely but not exclusively consisted of what will be referred to as a power chamber. A power chamber comprises a heat source reactor, an emissive spectrum converter (ESC), a photovoltaic element (PV) and a starter means. Some TPV power sources have proposed the use of heat recuperators.
A further description of the workings of TPV power generation is helpful in understanding the prior art, and provides a basis for understanding the present invention. A heat source reactor is any heat source capable of generating sufficient heat to bring to incandescence a narrow band thermal emitter. A thermal emitter, or emissive spectrum converter, when glowing hot will ideally be constructed to emit the majority of light in a narrow band, thus converting heat energy to light energy in a narrow spectrum. This is useful because the radiation is directed to a semiconductor photovoltaic material such as silicon. A semiconductor with a pn junction creating a potential barrier gives rise to a band gap that is a function of the material used. Silicon, with a band gap in electron volts of about 1.1 eV is equivalent to a wavelength of about 1150 nanometers. The more radiation that can be focused on the photovoltaic material with energy of 1.1 eV, the more electricity the photovoltaic material will produce. Of course, not all the heat energy will be usefully transferred to the photovoltaic material. This excess energy is lost as heat and in photons aimed away from the PV cells. An efficient TPV power source will capture excess heat in a recuperator and use this energy to precondition air acting as an oxidant and the fuel being fed to the heat source reactor.
Addressing specific embodiments of TPV technology, several U.S. Patents describe the major areas of advancement to date. For example, U.S. Pat. No. 4,584,426 teaches how "blackbody" radiation sources, such as the sun, are inefficient for producing the particular wavelength of light required by silicon photovoltaic power cells to produce electricity because the sun is a broadband radiation emitter. The patent teaches how a gas mantle can be prepared with a rare earth oxide such that when heated to incandescence, the mantle not only produces the majority of its light in a narrow band wavelength, but the spectrum is centered around that wavelength which is necessary for optimal production of electricity by silicon photovoltaic cells. The patent is typical of the macro-technology to which TPV technology has been applied.
Another example of TPV technology is the recuperator shown in U.S. Pat. No. 4,707,560 which teaches how preheat of the oxidant (air) can reduce the flame temperature required to heat the emissive spectrum converter. This makes the TPV device more efficient and slows the rate of fuel consumption.
Many improvements of macro TPV technology have concentrated on making a better gas mantle which is impregnated or coated with the rare earth oxides, such as U.S. Pat. Nos. 4,883,619, 4,975,044, and 5,057,162. Another macro improvement includes a ceramic fiber matrix heat source and emissive spectrum converter as taught by U.S. Pat. Nos. 5,356,487 and 5,360,490.
The overriding commonality between all of the patents described, and all others found by the inventors, is that TPV power sources are macro devices. That is to say that the physical dimensions of the components that comprise the power chamber are relatively large because the applications for which they were designed are household appliances such as hot water heaters and household electric power generators. The substantial technological problems of adapting TPV technology to micro-technology have until now precluded such adaptation.
What is needed is a way to provide portable power that has the lower energy density and safety of oil, but effective energy density greater than high explosives. The higher energy density would result not only in higher power output, but power output sustained over a longer period of time. In addition, a longer and preferably infinite shelf-life would result in less waste of resources, further reducing the impact of portable power use on the environment. Virtually instantaneous recharging of the power source would also be a definite advantage, as well as the elimination of equipment necessary for recharging. The power source should be compact and lightweight to replace batteries, and yet be rugged devices that would not suffer from the potential problems of high energy density power sources if mishandled. Finally, a wider range of fuel sources should be useable with the TPV power source.
As the above situations imply, it would also be an advantage over the prior art TPV power sources if the elements of the power chamber were miniaturized and ruggedized. It would also be an improvement over conventional batteries if a TPV power source could replace them without requiring more space than is currently occupied by the batteries, thus eliminating the cost of retrofitting portable devices for TPV power, and yet provide substantially greater energy that will last for a longer period of time.